Pressure sensing for a multi-arm catheter

ABSTRACT

A method for operating a medical probe includes pressing a distal end of the medical probe, which includes one or more arms that extend diagonally outward from a central shaft and have respective position transducers coupled thereto, against an intra-body surface, so as to cause the arms to exert pressure on the surface and bend with respect to the central shaft in response to the pressure. Positions of the respective position transducers coupled to the arms are measured, and the pressure exerted by the arms is estimated responsively to the measured positions.

FIELD OF THE INVENTION

The present invention relates generally to invasive probes, andspecifically to determining pressure exerted by a multi-arm catheter ona surface.

BACKGROUND OF THE INVENTION

A wide range of medical procedures involve placing objects, such assensors, tubes, catheters, dispensing devices and implants, within thebody. Position sensing systems have been developed for tracking suchobjects. Magnetic position sensing is one of the methods known in theart. In magnetic position sensing, magnetic field generators aretypically placed at known positions external to the patient. One or moremagnetic field sensors within the distal end of a probe generateelectrical signals in response to these magnetic fields, which areprocessed in order to determine the position coordinates of the distalend of the probe. These methods and systems are described in U.S. Pat.Nos. 5,391,199, 6,690,963, 6,484,118, 6,239,724, 6,618,612 and6,332,089, in PCT International Publication WO 1996/005768, and in U.S.Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and2004/0068178 A1, whose disclosures are all incorporated herein byreference.

In addition to catheters with a single distal tip discussed supra, U.S.Pat. No. 6,574,492, whose disclosure is incorporated herein byreference, discusses a catheter with a tuft of multiple resilient arms(also referred to as lobes) extending from the distal end of thecatheter. Each of the distal arms has a position sensor and one or moreelectrodes. There is also an additional position sensor in the distalend of the catheter, located at the base of the tuft.

When placing a probe within the body, it may be desirable to have thedistal tip(s) of the probe in direct contact with body tissue. Thecontact can be verified, for example, by measuring the contact pressurebetween the distal tip(s) and the body tissue. U.S. Patent ApplicationPublications 2007/0100332, 2009/0093806 and 2009/0138007, whosedisclosures are incorporated herein by reference, describe methods ofsensing contact pressure between the distal tip of a catheter and tissuein a body cavity using a force sensor embedded in the catheter. Thedistal tip of the catheter is coupled to the distal end of the catheterinsertion tube by a resilient member, such as a spring, which deforms inresponse to force exerted on the distal tip when it presses againstendocardial tissue. A magnetic position sensor within the cathetersenses the deflection (location and orientation) of the distal tiprelative to the distal end of the insertion tube. Movement of the distaltip relative to the insertion tube is indicative of deformation of theresilient member, and thus gives an indication of the pressure.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesa method for operating a medical probe, including:

-   pressing a distal end of the medical probe, which includes one or    more arms that extend diagonally outward from a central shaft and    have respective position transducers coupled thereto, against an    intra-body surface, so as to cause the arms to exert pressure on the    surface and bend with respect to the central shaft in response to    the pressure;-   measuring positions of the respective position transducers coupled    to the arms; and-   estimating the pressure exerted by the arms responsively to the    measured positions.

In some embodiments, the medical probe includes a catheter. In anembodiment, estimating the pressure includes verifying a physicalcontact between the arms and the surface. In a disclosed embodiment,estimating the pressure includes identifying that a given arm makes aphysical contact with the surface by detecting, using the measuredpositions, a change in a curvature of the given arm. In anotherembodiment, measuring the positions includes applying one or moremagnetic fields in a vicinity of the probe, receiving from the positiontransducers respective signals, which are generated by the positiontransducers responsively to the magnetic fields and are indicative ofthe respective positions of the position transducers, and calculatingthe positions based on the received signals.

In some embodiments, estimating the pressure includes calculating atleast one distance between at least one respective pair of the positiontransducers, and estimating the pressure responsively to the distance.In another embodiment, estimating the pressure includes calculating atleast one angle between at least one respective pair of the arms, andestimating the pressure responsively to the angle. In yet anotherembodiment, estimating the pressure includes calculating at least oneangle between the central shaft and at least one of the arms,respectively, and estimating the pressure responsively to the angle. Instill another embodiment, estimating the pressure includes applying tothe measured positions a pre-calibrated relation between the pressureand the positions.

In an embodiment, measuring the positions includes measuring a positionof an additional position transducer that is coupled to the centralshaft, and estimating the pressure includes assessing the pressureresponsively to the measured position of the additional positiontransducer. Estimating the pressure may include calculating at least onedistance between the additional position transducer and a respective atleast one of the position transducers, and estimating the pressureresponsively to the distance. In an embodiment, estimating the pressureincludes calculating at least one angle between the central shaft and arespective at least one of the arms, and estimating the pressureresponsively to the angle.

In some embodiments, the method includes displaying an image of the armsand the surface to an operator, and selecting a graphical feature usingwhich the arms are presented in the image responsively to the estimatedpressure. In an embodiment, the method includes selectively enablingsensing of signals by one or more electrodes coupled to at least one ofthe arms responsively to the estimated pressure.

There is additionally provided, in accordance with an embodiment of thepresent invention, including:

-   a medical probe, having a distal end including one or more arms that    extend diagonally outward from a central shaft and have respective    position transducers coupled thereto, the arms configured to press    against an intra-body surface so as to exert pressure on the surface    and bend with respect to the central shaft in response to the    pressure; and-   a processor, which is configured to measure positions of the    respective position transducers coupled to the arms, and to estimate    the pressure exerted by the arms responsively to the measured    positions.

There is also provided, in accordance with an embodiment of the presentinvention, a computer software product, operated in conjunction with amedical probe that includes one or more arms that extend diagonallyoutward from a central shaft and have respective position transducerscoupled thereto, the product including a computer-readable medium, inwhich program instructions are stored, which instructions, when read bya computer, cause the computer to measure positions of the respectiveposition transducers coupled to the arms, and to estimate the pressureexerted by the arms responsively to the measured positions.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic pictorial illustration of a medical system thatuses a multi-arm catheter, in accordance with an embodiment of thepresent invention;

FIG. 2 is a schematic side view showing details of the distal portion ofa multi-arm catheter, in accordance with an embodiment of the presentinvention; and

FIG. 3 is a flow diagram that schematically illustrates a method ofmeasuring pressure exerted by a multi-arm catheter on an intra-bodysurface, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Various diagnostic and therapeutic procedures, such as intracardiacelectrical mapping or cardiac ablation, use an invasive probe whosedistal tip is fitted with at least one electrode. The electrode istypically operated when the probe is pressed against intra-body tissue.In these procedures, it is usually important to maintain sufficientcontact pressure between the probe and the tissue in question. On theother hand, excessive pressure can have undesired effects on theprocedure, and in extreme cases even cause physical damage to thetissue.

Embodiments of the present invention provide methods and systems formeasuring the pressure that a multi-arm probe (e.g., a catheter) exertson tissue in a body cavity. In some embodiments, the distal end of amedical probe includes a central shaft and multiple arms that extenddiagonally outward from the central shaft. Each arm is fitted with aposition transducer. During a medical procedure, the distal end of thecatheter is pressed against an intra-body surface, so that the armsexert pressure on the surface. As a result of the pressure, the armsbend with respect to the central shaft. The positions of the positiontransducers in the arms are measured, and the contact pressure betweenthe arms and the surface is estimated based on the measured positions ofthe arms.

In some embodiments, an additional position transducer is fitted in thedistal end of the central shaft, in addition to the position transducersfitted in the multiple arms. The positions of the different positiontransducers are measured, including the additional position transducerin the central shaft, and the contact pressure is estimated based on theposition measurements.

Although the embodiments described herein refer mainly to multi-armprobes, some of the disclosed techniques can also be used in a probehaving a single arm fitted with a position transducer. In someembodiments, the magnitude of the contact pressure is estimated based onthe measured positions of the arms. In alternative embodiments, themeasured positions of the arms are used to verify physical contactbetween the arms and the surface, without necessarily measuring thepressure magnitude.

In some embodiments, the dependence of the measured positions on thepressure may be calibrated in advance. Coefficients calculated during acalibration procedure can be stored as a calibration matrix in anon-volatile memory that is coupled to the catheter. Then, when thecatheter is inside a body cavity such as a heart, the probe measurementsand the calibration coefficients may be used to verify that theelectrodes are in contact with the heart wall, and/or that the pressurebetween the electrodes and the heart wall is in the proper range forablation and/or sensing.

System Description

FIG. 1 is an illustration of a medical system 20 that uses a multi-armcatheter, which is constructed and operative in accordance with adisclosed embodiment of the invention. System 20 may be based, forexample, on the CARTO™ system, produced by Biosense Webster Inc.(Diamond Bar, Calif.). System 20 comprises a multi-arm probe 22, such asa catheter, and a control console 24. In the embodiment describedhereinbelow, it is assumed that probe 22 is used for diagnostic ortherapeutic treatment, such as mapping electrical potentials in a heart26 or performing ablation of heart tissue. Alternatively, probe 22 maybe used, mutatis mutandis, for other therapeutic and/or diagnosticpurposes in the heart or in other body organs.

An operator 28, such as a cardiologist, inserts multi-arm probe 22through the vascular system of a patient 30 so that a distal end 31 ofprobe 22 enters a chamber of the patient's heart 26. Operator 28advances probe 22 so that the distal end, comprising a plurality of arms32 extending from a central shaft 34, engages endocardial tissue at adesired location or locations. Probe 22 is typically connected by asuitable connector at its proximal end to console 24.

Console 24 uses magnetic position sensing to determine positioncoordinates of central shaft 34 and arms 32 inside heart 26. Todetermine the position coordinates, a driver circuit 36 in console 24drives field generators 38 to generate magnetic fields within the bodyof patient 30. Typically, field generators 38 comprise coils, which areplaced below the patient's torso at known positions external to patient30. These coils generate magnetic fields in a predefined working volumethat contains heart 26. Magnetic field transducers that are coupled toarms 32 of probe 22, and in some embodiments also to shaft 34, generateelectrical signals in response to these magnetic fields. (The distal endof probe 22, arms 32, shaft 34 and the different position transducersare shown in detail in FIG. 2 below.) A signal processor 40 in console24 processes the electrical signals in order to determine the positioncoordinates of arms 32 and possibly central shaft 34, typicallyincluding both location and orientation coordinates.

Processor 40 typically comprises a general-purpose computer, withsuitable front end and interface circuits for receiving signals fromprobe 22 and controlling the other components of console 24. Processor40 may be programmed in software to carry out the functions that aredescribed herein. The software may be downloaded to console 24 inelectronic form, over a network, for example, or it may be provided ontangible media, such as optical, magnetic or electronic memory media.Alternatively, some or all of the functions of processor 40 may becarried out by dedicated or programmable digital hardware components, orusing a combination of hardware and software elements.

An input/output (I/O) interface 42 enables console 24 to interact withprobe 22. Based on the signals received from probe 22 (via interface 42and other components of system 20), processor 40 drives a display 44 topresent operator 28 with a map 46 of cardiac electrophysiologicalactivity, as well as providing visual feedback regarding the position ofdistal end 31 in the patient's body, as well as status information andguidance regarding the procedure that is in progress.

Alternatively or additionally, system 20 may comprise an automatedmechanism (not shown) for maneuvering and operating probe 22 within thebody of patient 30. Such mechanisms are typically capable of controllingboth the longitudinal motion (advance/retract) of probe 22 andtransverse motion (deflection/steering) of central shaft 34 and arms 32.In such embodiments, processor 40 generates a control input forcontrolling the motion of probe 22 based on the signals provided by themagnetic field transducer in the probe. These signals are indicative ofboth the position of central shaft 34, and of force exerted on thecentral shaft (i.e., via arms 32), as explained further hereinbelow.

FIG. 2 is a schematic side view of distal end 31 of multi-arm probe 22,in accordance with an embodiment of the present invention. Specifically,FIG. 2 shows functional elements of central shaft 34 and arms 32. Distalend 31 comprises a tuft of three arms 32 extending diagonally outwardfrom central shaft 34. In the present example, arms 32 are substantiallysymmetrically arranged about a longitudinal axis 50 of central shaft 34,although any other suitable arm configuration can also be used. FIG. 2illustrates the arms substantially mutually spaced 120° apart about axis50. A radial dimple 52 is formed at a juncture between central shaft 34and each of arms 32, to enable the arms to bend backwards when pressingagainst a surface, such as the wall of heart 26.

Each of the arms comprises an electrode 54, which comes to contact withthe heart tissue and senses electrical signals in the tissue. Electrode54 may comprise, for example, a monopolar electrode or a bipolarelectrode useful for determining local electrical activity (e.g., localactivation time), and is typically made of a metallic material, such asa platinum/iridium alloy or another suitable material. Alternatively,multiple electrodes (not shown) along the length of each arm may be usedfor this purpose.

Each of the arms also comprises a position transducer 56, whichgenerates a signal to console 24 that is indicative of the positioncoordinates of its respective arm 32. An additional position transducer58 is fitted in central shaft 34 and generates a signal to console 24that is indicative of the position coordinates of the central shaft.Each of position transducers 56 and 58 may comprise one or moreminiature coils, and typically comprises multiple coils oriented alongdifferent axes. Alternatively, position transducers 56 and 58 maycomprise either another type of magnetic transducer, an electrode whichserves as a position transducer, or position transducers of other types,such as impedance-based or ultrasonic position transducers. AlthoughFIG. 2 shows a probe with a single position transducer in each of thearms, embodiments of the present invention may utilize probes with morethan one position transducer in any of the arms. When distal end 31 ispressed against body tissue during a medical procedure, processor 40 ofconsole 24 uses the signals received from position transducers 56, andsometimes transducer 58, to calculate the positions of the transducers.

In an alternative embodiment, the roles of position transducers 56, 58and magnetic field generators 38 may be reversed. In other words, drivercircuit 36 may drive magnetic field generators in position transducers56 and 58, so as to generate magnetic fields. Coils 38 may be configuredto sense the fields and generate signals indicative of the amplitudes ofthe components of these magnetic fields. In this embodiment, processor40 receives and processes the signals from coils 38 in order todetermine the position coordinates of central shaft 34 and arms 32within heart 26.

When pressing against a body cavity wall, the displacement of arms 32,either relative to each other and/or relative to central shaft 34, givesa measure of the deformation of each of the arms. Based on themeasurements received from position transducers 56 (and in someembodiments also transducer 58), processor 40 can calculate the pressureapplied by arms 32 against the wall of heart 26. Thus, the combinationof field generators 38 with position transducers 56 and 58 serves as apressure sensing system. This pressure sensing system reads the pressurecorrectly regardless of whether the pressure is exerted on arms 32head-on or at an angle.

In the present context, the term “estimating contact pressure” refersboth to quantitative pressure measurement and to verification ofphysical contact. In other words, processor 40 may estimate a numericalmagnitude of the pressure exerted by the arms, or verify whether or notthe arms come to physical contact with the heart surface. In the lattercase, processor 40 produces binary indications that indicate whether ornot the arms are in physical contact with the surface.

Although FIGS. 1 and 2 show a particular system configuration, othersystem configurations can also be employed to implement embodiments ofthe present invention, and are thus considered to be within the spiritand scope of this invention. For example, the methods describedhereinbelow may be applied using position transducers of other types,such as impedance-based or ultrasonic position transducers. The term“position transducer” as used herein refers to an element mounted onprobe 22 which causes console 24 to receive signals indicative of thecoordinates of the element. The position transducer may thus comprise areceiver on the probe, which generates a position signal to the controlunit based on energy received by the transducer; or it may comprise atransmitter, emitting energy that is sensed by a receiver external tothe probe. In some embodiments, some of the disclosed techniques can beused with a probe having only a single arm that is fitted with aposition transducer. Furthermore, the methods described hereinbelow maysimilarly be applied in mapping and measurement applications using notonly catheters, but also probes of other types, both in the heart and inother body organs and regions.

Contact Pressure Estimation Using Arm Position Measurements

As discussed supra, embodiments of the present invention provide methodsand systems for measuring the contact pressure between distal end 31 andthe intra-body tissue. In some embodiments, processor 40 processessignals from position transducers 56 in order to determine the positioncoordinates of arms 32, typically including both location andorientation coordinates. In some embodiments, processor 40 may use thecollected measurements to calculate one or more angles between arms 32.With reference to FIG. 2, processor 40 may calculate one or more angles60 between respective pairs of longitudinal axes 62 of arms 32.Alternatively or additionally, processor 40 may calculate one or moredistances 64 between pairs of position sensors 56.

In alternative embodiments, processor 40 processes signals received fromposition transducer 58, as well, in order to determine the positioncoordinates of central shaft 34 and arms 32. Processor 40 may use thecollected measurements to calculate an angle 66 between longitudinalaxis 50 of shaft 34 and longitudinal axis 62 of one of arms 32. Such acalculation can be performed for one or more of position transducers 56.In another embodiment, processor 40 estimates the curvature of a givenarm, or of multiple arms, based on the signals received from positiontransducers 56 and 58. The estimated curvature can also be used as anindicator of contact pressure or physical contact. In an exampleembodiment, processor 40 senses that a give arm makes physical contactwith the tissue by detecting a change in the curvature of the arm.

In further alternative embodiments, processor 40 may use the collectedmeasurements to calculate an angle 67 between a pair of arcs 68, whereeach arc 68 traverses position transducer 58 and one of position sensors56. Alternatively or additionally, processor 40 may calculate arespective distance 69 between position sensor 58 and a given positionsensor 56. Again, this calculation can be performed for one or more ofposition transducers 56.

To determine the exerted pressure, processor 40 may use coefficients(typically pre-calculated during a calibration procedure) to estimatethe pressure arms 32 are exerting on the intra-body tissue in questionbased on the calculated distances and/or angles.

In some embodiments, display 44 may present map 46 as a component of anovel user interface. For example, processor 40 may modify the way inwhich electrodes 54 in arms 32 are displayed on display 44, based on theestimated contact pressure. For example, if the contact pressure iswithin a predefined range that is regarded acceptable, the electrodescan be displayed using a certain color, icon or other graphical feature.If the contact pressure is outside the desired range, a differentgraphical feature will be used to display the electrodes. In anembodiment, processor 40 may refrain from displaying the electrodes ifthe contact pressure is out of range.

In some embodiments, processor 40 may enable sensing of electricalsignals by electrodes 54 only when there is sufficient contact pressureagainst the wall of heart 26 (so that the potential measurement islikely to be valid).

In some embodiments, processor 40 may estimate the contact pressureapplied by distal end 31 as a whole. In alternative embodiments,processor 40 may estimate and output the individual contact pressureexerted by each individual arm 32. For example, the processor may decidehow to display a given arm 32 on display 44, or whether to enablesensing by the respective electrode 54, based on the specific pressureexerted by that individual arm.

FIG. 3 is a flow diagram that schematically illustrates a method ofmeasuring the pressure exerted by probe 22 on an intra-body surface, inaccordance with an embodiment of the present invention. After operator28 positions probe 22 (step 70) in heart 26, processor 40 processes thesignals generated by position transducers 56 and 58 (step 72) andestimates the pressure exerted by arms 32 on endocardial tissue of heart26 based on the signals (step 74). As discussed supra, processor 40 mayderive the pressure based on parameters such as distances 64 and 69 orangles 60, 66 and 67. Processor 40 may use either a single parameter, ora combination of these or other parameters. Low pressure indicates thatthere may be inadequate contact between electrodes 54 and theendocardial tissue. High pressure may indicate that the electrodes arepressing too hard against the endocardial tissue.

Processor 40 checks whether the pressure measured at step 74 above iswithin a pre-specified acceptable range (step 76). If the contactquality is not within the specified range, console 24 may output anindication to display 44 of the pressure measured, and may issue analert if the pressure is too low or too high, thereby prompting operator28 to reposition probe 22 (step 78). The method then returns to step 70.Alternatively or additionally, the pressure indication may be used inclosed-loop control of an automated mechanism for maneuvering andoperating probe 22, as described hereinabove, to ensure that themechanism causes arms 32 to engage the endocardium in the properlocation, and with the appropriate pressure against the tissue.

If the contact pressure is within the specified range (step 76),processor 40 operates electrodes 54, e.g., collects map points (step80), and updates map 46. Finally, if operator 28 desires to collectadditional mapping data (step 82), then the method returns to step 70until the map is completed.

Although the operation of position transducers 56 and 58 is describedabove in the context of using a catheter for acquisition ofelectrophysiological mapping data, the principles of the presentinvention may similarly be applied in other therapeutic and diagnosticapplications that use invasive probes, both in heart 26 and in otherorgans of the body. For example, the devices and techniques that areimplemented in system 20 may be applied, mutatis mutandis, in gatedmapping of other physiological parameters, such as temperature orchemical activity, both in the heart and in other organs. Alteratively,system 20 may operate various other kinds of electrodes when the contactpressure is within range, e.g., apply ablation.

The corresponding structures, materials, acts, and equivalents of allmeans or steps plus function elements in the claims below are intendedto include any structure, material, or act for performing the functionin combination with other claimed elements as specifically claimed. Thedescription of the present disclosure has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimiting to the disclosure in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the disclosure. Theembodiment was chosen and described in order to best explain theprinciples of the disclosure and the practical application, and toenable others of ordinary skill in the art to understand the disclosurefor various embodiments with various modifications as are suited to theparticular use contemplated.

It is intended that the appended claims cover all such features andadvantages of the disclosure that fall within the spirit and scope ofthe present disclosure. As numerous modifications and changes willreadily occur to those skilled in the art, it is intended that thedisclosure not be limited to the limited number of embodiments describedherein. Accordingly, it will be appreciated that all suitablevariations, modifications and equivalents may be resorted to, fallingwithin the spirit and scope of the present disclosure.

1. A method for operating a medical probe, comprising: pressing a distalend of the medical probe, which includes one or more arms that extenddiagonally outward from a central shaft and have respective positiontransducers coupled thereto, against an intra-body surface, so as tocause the arms to exert pressure on the surface and bend with respect tothe central shaft in response to the pressure; measuring positions ofthe respective position transducers coupled to the arms; and estimatingthe pressure exerted by the arms responsively to the measured positionsby calculating at least one distance between at least one respectivepair of the position transducers, and estimating the pressureresponsively to the distance.
 2. The method according to claim 1,wherein the medical probe comprises a catheter.
 3. The method accordingto claim 1, wherein estimating the pressure comprises verifying aphysical contact between the arms and the surface.
 4. The methodaccording to claim 1, wherein estimating the pressure comprisesidentifying that a given arm makes a physical contact with the surfaceby detecting, using the measured positions, a change in a curvature ofthe given arm.
 5. The method according to claim 1, wherein measuring thepositions comprises applying one or more magnetic fields in a vicinityof the probe, receiving from the position transducers respectivesignals, which are generated by the position transducers responsively tothe magnetic fields and are indicative of the respective positions ofthe position transducers, and calculating the positions based on thereceived signals.
 6. (canceled)
 7. The method according to claim 1,wherein estimating the pressure comprises calculating at least one anglebetween at least one respective pair of the arms, and estimating thepressure responsively to the angle.
 8. The method according to claim 1,wherein estimating the pressure comprises calculating at least one anglebetween the central shaft and at least one of the arms, respectively,and estimating the pressure responsively to the angle.
 9. The methodaccording to claim 1, wherein estimating the pressure comprises applyingto the measured positions a pre-calibrated relation between the pressureand the positions.
 10. The method according to claim 1, whereinmeasuring the positions comprises measuring a position of an additionalposition transducer that is coupled to the central shaft, and whereinestimating the pressure comprises assessing the pressure responsively tothe measured position of the additional position transducer.
 11. Themethod according to claim 10, wherein estimating the pressure comprisescalculating at least one distance between the additional positiontransducer and a respective at least one of the position transducers,and estimating the pressure responsively to the distance.
 12. The methodaccording to claim 10, wherein estimating the pressure comprisescalculating at least one angle between the central shaft and arespective at least one of the arms, and estimating the pressureresponsively to the angle.
 13. The method according to claim 1, andcomprising displaying an image of the arms and the surface to anoperator, and selecting a graphical feature for the arms presented inthe image responsively to the estimated pressure.
 14. The methodaccording to claim 1, and comprising selectively enabling sensing ofsignals by one or more electrodes coupled to at least one of the armsresponsively to the estimated pressure.
 15. Apparatus, comprising: amedical probe, having a distal end comprising one or more arms thatextend diagonally outward from a central shaft and have respectiveposition transducers coupled thereto, the arms configured to pressagainst an intra-body surface so as to exert pressure on the surface andbend with respect to the central shaft in response to the pressure; anda processor, which is configured to measure positions of the respectiveposition transducers coupled to the arms, and to estimate the pressureexerted by the arms responsively to the measured positions, theprocessor being configured to calculate at least one distance between atleast one respective pair of the position transducers, and to estimatethe pressure responsively to the distance.
 16. The apparatus accordingto claim 15, wherein the medical probe comprises a catheter.
 17. Theapparatus according to claim 15, wherein the processor is configured toverify a physical contact between the arms and the surface.
 18. Theapparatus according to claim 15, wherein the processor is configured toidentify that a given arm makes a physical contact with the surface bydetecting, using the measured positions, a change in a curvature of thegiven arm.
 19. The apparatus according to claim 15, wherein theprocessor is configured to cause one or more field generators to applyone or more magnetic fields in a vicinity of the probe, to receive fromthe position transducers respective signals, which are generated by theposition transducers responsively to the magnetic fields and areindicative of the respective positions of the position transducers, andto calculate the positions based on the received signals.
 20. (canceled)21. The apparatus according to claim 15, wherein the processor isconfigured to calculate at least one angle between at least onerespective pair of the arms, and to estimate the pressure responsivelyto the angle.
 22. The apparatus according to claim 15, wherein theprocessor is configured to calculate at least one angle between thecentral shaft and at least one of the arms, respectively, and toestimate the pressure responsively to the angle.
 23. The apparatusaccording to claim 15, wherein the processor is configured to estimatethe pressure by applying to the measured positions a pre-calibratedrelation between the pressure and the positions.
 24. The apparatusaccording to claim 15, and comprising an additional position transducercoupled to the central shaft, wherein the processor is configured tomeasure a position of the additional position transducer, and toestimate the pressure responsively to the measured position of theadditional position transducer.
 25. The apparatus according to claim 24,wherein the processor is configured to calculate at least one distancebetween the additional position transducer and a respective at least oneof the position transducers, and to estimate the pressure responsivelyto the distance.
 26. The apparatus according to claim 24, wherein theprocessor is configured to calculate at least one angle between thecentral shaft and a respective at least one of the arms, and to estimatethe pressure responsively to the angle.
 27. The apparatus according toclaim 15, wherein the processor is coupled to display an image of thearms and the surface to an operator, and to select a graphical featurefor the arms presented in the image responsively to the estimatedpressure.
 28. The apparatus according to claim 15, wherein the probecomprises one or more electrodes coupled to at least one of the arms,and wherein the processor is configured to selectively enable sensing ofsignals by the electrodes responsively to the estimated pressure.
 29. Acomputer software product, operated in conjunction with a medical probethat includes one or more arms that extend diagonally outward from acentral shaft and have respective position transducers coupled thereto,the product comprising a computer-readable medium, in which programinstructions are stored, which instructions, when read by a computer,cause the computer to measure positions of the respective positiontransducers coupled to the arms, and to estimate the pressure exerted bythe arms responsively to the measured positions by calculating at leastone distance between at least one respective pair of the positiontransducers, and estimating the pressure responsively to the distance.